The present invention relates to a coating composition capable of forming, in short cure times and at low temperatures, a transparent protective film having appropriate surface hardness, excellent heat resistance, excellent weathering resistance and good adhesion to the surface of plastic materials and the like.
Plastics have a number of excellent properties that are not found in other materials in that they are light, tough and easy to process. Because of such advantags, plastics have been applied in a variety of fields, and further, due primarily to the development of engineering plastics, they are now being applied even in fields where application thereof has heretofore been impossible. However, while plastics have the above mentioned advantages, they also have disadvantages such as low surface hardness, poor heat resistance, poor weathering resistance and solvent resistance.
In order to overcome such problems, various attempts have been made to develop compositions which can be applied onto the surface of a plastic substrate to improve the surface hardness thereof. Among such compositions, there has been suggested a colloidal silica containing silicone coating composition which is characterized by excellent mechanical properties as a cured film (Japanese Patent Publication Kokai No. 56/125466 (1981)). However, in the coating of plastics using such a composition, the cure times are undesirably long, about 1 hour of heating, and the cure temperatures are undesirably high, about 120.degree. C. Accordingly, use of such a composition is sometimes difficult depending on the heat resistance of the plastic substrate and the expense of curing equipment particularly in the case of equipment for continuous curing.
As a method to overcome the disadvantages, there has been proposed a method to add a fatty acid salt of an alkali metal as a curing catalyst to the above mentioned composition. However, in this method, there are problems that the heat resistance and the weathering resistance of the resulting film are insufficient. The film becomes opaque easily upon exposure to high temperature or upon exposure to prolonged outdoor weathering. Further, if too much curing catalyst is utilized, the film may crack during heat cure.
On the other hand, there is a method where an ammonium hydroxide or amines are used as the curing agent for the above mentioned composition. However, in such a method the storage stability of the composition obtained is poor and orange peel may occur upon heat curing. This makes it difficult to obtain a transparent film. Because of such reasons, it has been impossible to use said coating agent in a continuous dip coating and curing line.
In addition to the above methods, there has been proposed a method, wherein the condensate of alkyltrialkoxysilane hydrolysate is cured using tetramethylammonium hydroxide catalyst without incorporating colloidal silica into the composition (Japanese Patent Pulication Kokai No. 57/38863 (1982)). However, the strength of the film obtained by this method is weaker than the film obtained in combination with colloidal silica. In an abrasion resistance test, the former film shows only 1/2 to 1/8 of the surface hardness of the latter. Moreover, in this method the curing requires long cure times and high temperature such as from 4 to 5 hours at 90.degree. to 100.degree. C.
Thus, as described above, it has heretofore been impossible to obtain a film having appropriate surface hardness, excellent heat resistance, excellent weathering resistance and good adhesion to plastic substrates using low cure temperatures and short cure times.